CHAPTER IV. 



CLASSIFICATION OF IGNEOUS ROCKS. 



TTWERY student of geology experiences the greatest difficulty in giving 

 [j names to rocks, and this difficulty is shared to a great extent by those 

 who have had a considerable amount of experience. It is tolerably easy 

 now-a-days to describe a rock, to ascertain and record its chemical and 

 mineralogical composition and textural characteristics ; but it is by no 

 means so easy to recognise its position in a natural system of classification. 



The difficulty arises partly in consequence of the nature of the subject 

 and partly in consequence of the absence of agreement amongst petrographers 

 as to the first principles upon which petrographical classification should be 

 based. The object of the present chapter is not to propose any new system, 

 but to consider the question of classification from a general point of view 

 and to call attention to certain points which strike the author as being of 

 considerable importance. 



In the first place we may ask ourselves the question : Is there any 

 one natural system of rock- classification ? This is by no means easy to answer, 

 but we may at least call attention to certain radical differences between 

 rocks and the objects with which the workers in the other branches of the 

 natural history sciences have to deal. A consideration of these differences 

 will show that the biologist and the mineralogist have certain advantages 

 over the petrographer so far as matters relating to classification are 

 concerned. 



Modern biology recognises the fact that the natural system of classifi- 

 cation, so far as living objects are concerned, is the one which best expresses 

 the mutual relations of organisms from the point of view of evolution. In 

 doubtful cases the biologist applies, where possible, the test of embryological 

 development, and this test is accepted on all hands as conclusive. It may be 

 urged that the recognition of the principle of " descent with modification" 

 is of comparatively modern date, and that its introduction has not seriously 

 disturbed the old established system. This is of course true, but then it 

 must be remembered that the unanimity which existed amongst biologists 

 before the days of DARWIN, on matters relating to classification, was the 

 manifestation of the great truth which he was mainly instrumental in 

 establishing. 



It thus appears that a principle of the utmost generality underlies and 

 furnishes, as it were, the justification of biological classification. Now in 

 petrography there is no generally recognised principle in any way comparable 



